Ankle stiffness of standing humans in response to imperceptible perturbation: reflex and task‐dependent components.

1. It has been demonstrated that subjects can alter the reflex stiffness of the elbow and wrist in response to imperceptibly slow perturbations applied through a complaint coupling. We used this technique to measure ankle stiffness in standing subjects as a means of examining reflex activity. 2. During unperturbed stance, a linear relationship between ankle torque and ankle angle is expressed as a load stiffness. The load stiffness predicted from a subject's measured physical dimensions corresponds closely with the value measured by standing the subject on a force platform. 3. Slow perturbations were applied at waist level, through a spring, to standing subjects. The perturbations caused sway similar in magnitude and rate to the sway of normal stance. Ankle stiffness was measured during the period when the perturbations were unperceived. The contribution to ankle stiffness of reflexes that use visual information was assessed by eye closure. The ability of reflexes based on sensory information from the legs to maintain upright posture was assessed when subjects balanced a load equivalent to their own body, in a situation where neither visual nor vestibular information could assist. Ankle stiffness was measured while the load was perturbed. 4. The results show that a simple mechanical model of stance predicts the torque‐angle relationship at the ankle. This relationship determines the minimal ankle stiffness required to stand, and reflex muscle stiffness is a necessary component of this ankle stiffness. Visual, vestibular and lower limb sensorimotor reflexes each contribute to ankle stiffness; however, the local sensory reflexes alone are sufficient to stand. For responses to unperceived perturbations, standing subjects can alter their reflex ankle stiffness according to intentional set.